<?php
    $city = $this->formSelect('city',null,array('onchange'=>"loadCbo(this.value)"),$this->slbCity);
    $options[] = "Select an item in city";
    $district = $this->formSelect('city',null,null,$options);
    $urlDistrict = $this->url(array('module'=>'training','controller'=>'ajax','action'=>'district'));
?>
City:
<div id="cbo-city" class="div">
    <?php echo $city;?>
</div>
District:
<div id="cbo-district"  class="div">
    <?php echo $district;?>
</div>
<script type="text/javascript">
    function loadCbo(id){
        if(id > 0 ){
            $.get('<?php echo $urlDistrict . '/id/';?>'+id,getData);
        }
    }

    function getData(data){
        $('div#cbo-district').html(data);
    }
</script>
<style type="text/css">
.div{
    border: solid 1px #ccc;
    padding: 10px;
    margin: 5px;
}
</style>